One type of gas turbine engine igniter design has a ceramic surface extending between a center electrode and a ground electrode at a spark gap. The ground electrode is annular and coaxial with the center electrode. An ignition system supplies high energy high voltage pulses to the igniter. The system may include a capacitor which is charged as an applied voltage increases. When the voltage in the capacitor becomes sufficiently large to cause a spark discharge at the igniter, the energy stored in the capacitor is discharged to the spark gap. The fuel igniting spark travels over the ceramic surface at a spark gap.
Because of weight considerations in the aircraft industry, various techniques have been employed to reduce the voltage needed to produce a spark of the required energy and thereby to reduce the size and weight of the ignition system. The size and weight of the ignition system can be reduced by reducing the voltage requirement of the igniter. One way to reduce the igniter's voltage requirement is to place a semiconducting ceramic insert across the spark gap or to apply a semiconducting or engobe surface coating on the insulator at the spark gap. This establishes a large value resistance in parallel with the spark gap which is lower than the resistance of air. Consequently, a spark of the required energy will occur at a lower voltage. While decreasing the voltage needed to initiate a spark discharge in an igniter, a semiconducting or engobe surface coating can cause certain problems at the spark gap. For example, the semiconducting or engobe coating is prone to wear under normal use. One type of engobe is produced by adding sufficient metallic copper powder or copper dioxide to an electrically insulating glaze to obtain the desired resistivity. Under severe conditions, a spark gap surface completely coated with such an engobe can become elemental copper, resulting in a short circuit between the electrodes. Accordingly, there is a need for an igniter having a longer lasting semiconducting coating on the ceramic which is less likely to breakdown into a short circuit across the spark gap under harsh operating conditions.